1. Field of the Invention
The present invention relates generally to the control of sootblowers in a fossil fueled power plant and more particularly to power plant applications systems using a graphical programming environment in combination with a set of rules to activate sootblowers.
2. Description of the Prior Art
Sootblowing is necessary to control fouling of furnace tube surfaces from by-products of the combustion process (primarily slag and soot). Generally, this is from the combustion of fossil fuel (for example, coal or oil) in a power plant for generating electricity or process steam. Sootblowing involves the removal of slag and soot with high-velocity jets of air, steam, or water.
However, there are performance penalties associated with sootblowing. The air, steam and water all impart a heat rate penalty (lower efficiency) to the power plant as they add material to the combustion process that will be heated and rejected as waste heat. In addition, the equipment itself requires power to operate, and the medium must be cleaned. Finally, the interjection of the material may lead to control issues either through sudden cooling (especially with the use of water) or by sudden temperature excursion caused by a newly cleaned surface taking in heat more quickly than the reaction time of the control system.
Additional costs occur from under-blowing, with sections that can no longer be cleaned because of excessive soot buildup. This may lead to excessively high Reheat (RH) and Superheat (SH) and Economizer temperatures as the heat not being removed from the lower water walls overwhelms the ability of spray flows or other temperature control mechanisms to maintain a proper setpoint resulting in heat being vented to the atmosphere rather than being used by the system. Conversely, over-blowing a hot surface leads to erosion and the thermal stress of the heat transfer surface. These conditions may lead to tube leaks, which at a minimum lowers the efficiency of the unit, and at worst may cause the unit to be shut down to fix the leak.
The control logic for sootblower sequencing is normally programmed into a programmable logic controller (PLC). Most modern systems will have a personal computer with a graphical user interface (GUI) for modifying sequences. Various methods of supervisory control have been implemented for the timing of activation of the sootblowers. The PLC's usually activate blowers based on timers. When an operator starts such a sequence, the system will run each blow at a predetermined time. The operator may hold the sequence, abort the sequence, or restart the sequence. Boiler engineers interact with operations to continually alter the sequences based on inspection reports identifying soot build-up (requiring more frequent blowing) or areas that appear overblown (requiring less frequent blowing). The user may have a number of predetermined sequences. These often are designed to clean specific heat exchange sections (e.g. water wall, reheat pendants, superheat pendants, economizers, and the like). Some sequences may be developed for use with different fuel types. However, pre-determined time blowing is sub-optimal as it is blind to existing slag and soot build-up.
The next generation of supervisory control introduced into the industry involved intelligent sootblowing (ISB). Generally a combination of cleanliness models, heat transfer models, heat flux models, statistical models, neural networks, expert systems, and in-situ furnace instrumentation attempts to enable plant operators to optimize sootblowing based on actual boiler operating conditions. These methods have been somewhat successful in improving the overall performance of sootblowing operations, but have common issues with maintaining additional instrument and calculations. The methods also tend to be complex leading to black boxes that prevent end-users for tuning the system further.
Several inventions are known in the prior art:
U.S. Pat. No. 5,181,482 is one of the earlier patents to describe the use of predictive models to make predictions and use this as guidance in adjusting a sootblowing sequence. The models were used to calculate the rate of fouling on specific sections of the boiler. This information is presented to the boiler operator to assist in enhancing the boiler efficiency and maintain steam temperatures within established control ranges.
U.S. Pat. No. 6,425,352 focuses on achieving desired conditions of specific surface conditions. While cleanliness models can generate this type of data, this patent broadens the concept to include any parameter indicative of the extent of deposits remaining on the surface. This parameter becomes the measure of desired condition to be achieved. The patent allows for a feedback to adjust the aggressiveness (frequency) of a blower if the desired factor is not reached or to decrease (the frequency) if the target is easily met.
The concept is further extended in U.S. Pat. No. 6,325,025 whereby a direct sensor associated with said surface is used to determine a parameter indicative of the condition of said surface. The main limitation with this concept is the focus on specific section data, without a solid method to look at the global impact of changes.
U.S. Pat. Nos. 7,458,342 and 6,736,089 describe systems that specifically utilize higher order models such as cleanliness factor, neural networks, and mass energy balance equations. These models look to balance out some of the factors neglected in earlier methods. Whether through use of direct or indirect controllers, the goal is to optimize these settings (e.g. cleanliness factor). They all require a number of inputs to be useful. The need for a number of variable results in greater inaccuracies, hidden sensitivities in the data, and increased need for methods to accommodate a loss of signals. Furthermore, these system assumes there are desired cleanliness levels for given sections of the boiler that are static. These systems schedule blowers as thresholds are crossed with a resulting a queue of blowers that will blow in the sequence they are queued.
U.S. Pat. No. 6,928,937 describes a method for determining when a furnace is to be cleaned with the view to the impact on thermal efficiency of the steam cycle of the furnace. The method looks at how blowers impact the overall efficiency by looking at the deviations the blowers had in the past and using them to guide activation in the future. The method results in activation of sequences of blowers or the queuing of blowers to be activated in a sequence.
U.S. Pat. No. 7,891,323 adds the novel concept of using the weight of a heat exchanger as an evaluation tool for triggering a sootblowing sequence. The method offers a direct way to measure the actual weight of slag and soot buildup per section of heat exchange surfaces that are hung from the infrastructure of the boiler.
In the U.S. Pat. No. 7,890,214, a method is described that addresses some of the limitation with model based sootblowing schemes previously described, though it still relies on heat absorption data that must be calculated somewhere in the system. The patent describes a statistical process control system analyzing the distribution of the heat absorption data as well as various parameters of the heat absorption distribution and using this to readjust the soot blowing operation. Besides leading to automatic adjustments in sequences, the method can also be used to detect permanent slagging conditions.
U.S. Pat. No. 7,890,197 describes a multi-model system for determining the operating sequences. The patent describes a set of sequences generated by a clean model and set of sequences generated by a model for dirty unit. These models are used based on the clean or dirty state for the specific heat exchange section.
Other types of approaches have been used including expert systems, but these again rely on cleanliness and heat flux calculations to allow logic to be developed that modifies existing sequences to better reflect the actual amount of soot/slag removed by individual blowers.
As can be seen from the prior art, a number of different techniques have evolved over time to improve upon the basic time sequenced PLC activation logic. The models themselves have become increasing sophisticated and use multiple approaches to determining the cleanliness (dirtiness) of various heat exchange sections.
It would be extremely advantageous to have a sootblowing system and method that does not rely on sophisticated models but does not preclude them either; is not expert system based per se, but rather operates off a reasonably simple set of rules accommodating all the various modes of operation. It would also be advantageous to have a system and method where the user imparts his knowledge of the key control variables that by definition are going to have high availability such as reheat (RH) and superheat (SH) temperature, RH and SH spray flows, Economizer Temperatures and the like while allowing the system to include other direct measurements such as a delta pressure across heat exchange sections as an early indication of fouling.